单例模式

      设计模式大家都很熟悉，今天来记录下最基础也最简单的单例模式

      单例模式分为两类，一是饿汉式，另外就是相对的懒汉式

      想来看看饿汉式

      常见的实现方式如下：

public class SingletonHungry {
    private static SingletonHungry instance = new SingletonHungry();

    private String token;

    private SingletonHungry() {
        token = System.currentTimeMillis() + "";
    }

    public static SingletonHungry getInstance() {
        return instance;
    }

    public String getToken() {
        return token;
    }
}

新增token来验证线程安全

来个线程安全的

public class SingletonHungrySafe {

    private static String token;

    //jvm加载时会实例化
    private static class Holder {
        private static SingletonHungrySafe INSTANCE = new SingletonHungrySafe();
    }

    private SingletonHungrySafe() {
        token = System.currentTimeMillis() + "";
    }

    public static final SingletonHungrySafe getInstance() {
        return Holder.INSTANCE;
    }

    public String getToken() {
        return token;
    }
}

可以保证在各个线程获取Holder.INSTANCE变量之前完成。在保证线程安全的同时，又可以延迟实例化，并且没有降低并发性

接下来看看懒汉式，相对的，懒汉式是用时才加载，延后性。
先来个线程不安全的

public class SingletonLazy {
    private static SingletonLazy instance;

    private SingletonLazy() {
    }

    public static SingletonLazy getInstance() {
        if (instance == null) {
            instance = new SingletonLazy();
        }
        return instance;
    }
}

线程安全的

public class SingletonLazySafe {
    private volatile static SingletonLazySafe instance;

    private SingletonLazySafe() {
    }

    public static SingletonLazySafe getInstance() {
        if (null == instance) {
            synchronized (SingletonLazySafe.class) {
                if (null == instance) {
                    instance = new SingletonLazySafe();
                }
            }
        }
        return instance;
    }
}

双重检查加锁，保证线程安全。

同时，我们可以对这些进行测试

public class SingletonTest {

    public static void main(String[] args) throws InterruptedException {
        for (int i = 0; i < 1000; i++) {
            Single single = new Single();
            single.start();
        }
    }

    static class Single extends Thread {

        @Override
        public void run() {
            checkSingleHungry();
        }
    }

    private static void checkSingleHungrySafe() {
        SingletonHungrySafe singletonHungrySafe = SingletonHungrySafe.getInstance();
        System.out.println("Thread id=" + Thread.currentThread().getId() + "token:" + singletonHungrySafe.getToken());
    }

    private static void checkSingleHungry() {
        SingletonHungry singletonHungry = SingletonHungry.getInstance();
        System.out.println("Thread id=" + Thread.currentThread().getId() + "token:" + singletonHungry.getToken());
    }
}

验证线程安全问题